While loop in Haskell with a condition

haskell, loops, while-loop

Solution

import Control.Monad

while :: (a -> Bool) -> (a -> IO a) -> a -> IO a
while praed funktion x
    | praed x   = do
        y <- funktion x
        while praed funktion y
    | otherwise = return x


power2 :: IO ()
power2 = do
    putStr "Please enter a number."
    i <- getChar
    let praed x = x /= ' '
    let f x = do
        putChar x
        getChar
    while praed f '?'
    return ()

Some notes:

- Using `if x then True else False` is redundant, it's equivalent to just `x`.

- Similarly `if x == True ...` is redundant and equivalent to `if x ...`.

You need to distinguish between `IO` actions and their results. For example, if yo do

do
    i <- getChar
    ...

then in ... `i` represents the result of the action, a character, so `i :: Char`. But `getChar :: IO Char` is the action itself. You can view it as a recipe that returns `Char` when performed. You can pass the recipe around to functions etc., and it is only performed when executed somewhere.

Your `while` called `funktion` twice, which probably isn't what you intend - it would read a character twice, check the first one and return the second one. Remember, your `funktion` is an action, so each time you "invoke" the action (for example by using `<- funktion ...` in the `do` notation), the action is run again. So it should rather be something like

do
    y <- funktion x
    f <- praed y
    -- ...

(My code is somewhat different, it checks the argument that is passed to it.)

Problem

Original source